1. Field of the Invention
The present invention relates to ignition systems for spark ignition type engines and more particularly to an improved ignition system which is capable of reducing wear of spark plug electrodes.
2. Description of the Prior Art
Recently, the spark plugs used in automobiles have been required to produce strong spark discharges due to the introduction of exhaust emission controls and use of thiner air-fuel mixtures and hence wear of the plugs has becomes increasingly severe. Moreover, the Official prescription of plug maintenance has been predicted in certain countries and there has been an increasing demand for an ignition system designed to reduce wear of spark plug electrodes.
In known ignition systems of the type employing a high voltage generator, the discharge time of the spark plug amounts to about 2 ms and this discharge time is practically held constant even at higher engine speeds. In such a case, the spark plug maintains the discharge even after the top dead center (TDC) so that if the ignition occurs before the TDC, a combustion is started at around the TDC and consequently the wasteful discharge is continued even after the combustion has already started. When a combustion is taking place in a combustion chamber, both the temperature and pressure in the combustion chamber become very high and consequently the discharging at the spark plug in such a condition has the disadvantage of greatly promoting wear of the plug electrodes.
To overcome this disadvantage, a method has been proposed in U.S. Pat. No. 3,896,776 in which the primary winding of the ignition coil is again energized at a specified time instant so as to interrupt the undesired ignition spark. A disadvantage of this method is that since the ignition spark is continued until the specified time instant, if the ignition is accomplished by that time instant, the plug wear reducing effect will be deteriorated. Further, the application of this method to a conventional ignition system has the disadvantages of requiring a considerable modification of the system and excessively increasing the energization time of the primary winding resulting in overheating the ignition coil.